In the production of corrugated board a multiplicity of grades of linerboard and medium are used as well as a variety of flute formations. This requires frequent adjustment of the glue machine to maintain quality production of corrugated board. The basic concept of the present invention allows the glue machine to adapt to this changeable environment of corrugated production automatically without operator intervention required after startup of the corrugator.
In the first step of corrugated board production, a machine called a singlefacer is used to flute a given grade of medium (paper) between a pair of corrugating rolls machined to a specified profile. This fluted medium is then bonded to a liner of various grades of paper using a starch based adhesive. After combining the medium and liner in this fashion, the resulting singleface web progresses to a bridge storage area where latent heat that has been applied to the medium and liner continue to cure the starch based adhesive securing the bond.
The next step in the corrugating process takes the singleface web produced at the singlefacer and combines it with another (bottom) liner. This bottom liner becomes the exterior surface of a corrugated container and is usually a finer grade of paper. This surface (the box exterior) will normally have flexographic printing applied in the process of creating a finished box. Alternatively, a preprinted liner can be used or a label can be affixed to the outer surface of the corrugated box blank to create the finished box. Bonding the second liner, known as the “doubleface” liner, requires an even application of adhesive onto the medium flute tips across the full width of the singleface web.
The application of adhesive to the singleface web flute tips occurs in a machine referred to as a glue machine. A primary feature of this machine is a glue applicator roll, which may have an engraved surface. A film of adhesive is picked up by the glue applicator roll as it rotates through a glue pan filled with starch based adhesive. The adhesive is metered onto the glue roll, typically using a contra rotating metering roll, so that a consistent glue film thickness is applied across the working width of the glue applicator roll surface. Other methods of metering are applicable, such as those defined in U.S. Pat. No. 6,068,701 dated May 30, 2000.
The glue applicator roll usually runs at a speed some small percentage less than the speed of the singleface web passing in contact with the roll, commonly 95%-98% of singleface web speed. The roll underspeed is crucial to achieve starch application centered on the flute tip allowing for proper bonding to the doubleface liner. Maintaining proper glue roll rotational speed is achieved through the use of a drive with a regenerative feature. This regenerative feature is critical to maintaining the proper speed ratio between the singleface web and glue applicator roll surface.
A glue machine can be equipped with a rider roll designed to bring the flute tips of the singleface web into intimate contact with the adhesive film on the glue applicator roll. The rider roll must be positioned to create an adjustable gap between it and the glue applicator roll through which the singleface web passes. This gap ensures the singleface web flute tips pick up the desired amount of adhesive. Improper setting of the rider roll to glue applicator roll gap can create two undesirable conditions. If the gap setting is too loose, areas along the flute tips may pick up too little starch or no starch at all. This will result in the formation of a blistered and undesirable exterior surface of the corrugated box. If the rider roll to glue roll gap is set too tight, the singleface web passing through this nip will be deformed and damaged. This compression of the board past its elastic range can result in a significant loss in the mechanical strength of the corrugated box deeming it unacceptable to its application. Significant singleface web compression also results in excessive starch application with several negative effects beyond the cost of the starch consumed. For example, excess starch application will cause wash boarding that is difficult to print on and that shows up as undesirable striated lines through a preprinted or labeled surface. Excessive starch application also results in increased energy consumption required to gel the starch and drive the moisture from the glue line.
Rider roll/glue applicator roll gap setting has been normally left to the operator on prior technology glue machines. This can lead to improper gap setting, particularly on corrugators that involve a lot of paper grade changes. It is desirable, therefore, to implement a means of automatic adjustment of the rider roll to glue applicator roll gap.
Automatic rider roll gap setting means have been described in the prior art. Several contact and non-contact means have been disclosed in the literature for direct measurement of singleface web caliper upstream of the glue machine for purposes of command positioning of the rider roll gap. U.S. Pat. No. 4,360,538 discloses, for example, a contact singleface web caliper sensing device that derives a signal that is used to adjust the rider roll gap setting to achieve a desirable compression of the singleface web between the rider roll and glue applicator roll. US Patent Publication 2008/0317940 A1 discloses several non-contact singleface web flute height sensing techniques that use a curtain of visible, infrared or ultraviolet light or laser beams. Any of these upstream flute height measurement techniques, when used in conjunction with a rider roll to glue applicator roll gap measurement, can be used for automatic setting of the desired gap. Also disclosed is a contact automatic singleface web caliper sensor as shown in FIG. 10 of the same publication. This means of sensing singleface web caliper will be discussed in more detail in ensuing paragraphs. All of these contact and non-contact methods for singleface web flute height measurement and subsequent rider roll gap setting add complexity and require absolute calibration of the singleface web flute height sensing means as well as the rider roll gap adjustment hardware that can drift out of tune with time creating a maintenance issue.
Concepts have been described in the literature for use of pressure loading of the rider roll to force the flutes of the singleface web into contact with the glue applicator roll. Means of actuation and sensing of pressure force, for example, are described in U.S. Pat. No. 6,620,455 B2. The pressure loading of the rider roll causes deflection of the singleface web flutes as clearly shown in FIG. 4 of US Patent Application Publication 2008/0317940 A1. There are several problems with the pressure loading concepts. First, the required pressure loading must be empirically determined based upon the strength of the flute tip. The flute tip strength varies considerably with type of flute formation as well as within a flute type as a function of the medium basis weight and even the manufacture of the medium. As a consequence it is difficult to select the desired pressure setting without getting too much or too little deflection of the flute tips. Too little deflection can cause poor starch adhesive transfer, and too much deflection can cause permanent crush to the flute tips causing degradation in the quality of the corrugated board manufactured. In addition, the means of sensing and controlling the pressure are complex and suffer from performance issues related to the bad environment of the glue machine. Starch adhesive is caustic and, as is well known in the art, splashes about the glue machine contaminating operating mechanisms and requiring frequent clean up. The contamination can affect the precision of the pressure loading mechanisms making them difficult to use in practice.
As a consequence, there is still a need in the art for an improved means of automatically controlling the rider roll to glue applicator roll gap to a precise setting to achieve sufficient and necessary adhesive transfer to the singleface web flute tips without compressing the flutes such that permanent damage occurs. Furthermore, it is desirable to achieve these objectives without unduly complex addition of mechanical mechanisms that require maintenance and frequent cleaning to keep them operating. In particular it is desirable to avoid requirement for periodic absolute calibration of measuring and controlling sensors to keep them functional and operable.